The present invention is directed to optical communications. More particularly, the present invention is directed to an optical switch for an optical network.
The enormous increase in data traffic, largely due to the growth in Internet traffic, has spurred rapid growth in broadband communication technologies. Fiber optics, which offers the largest bandwidth of any communication system, is the medium of choice for carrying the multitude of data now being sent through networks. While fiber can theoretically carry over 50 terabits per second, current optical communication systems are limited to 10 gigabits per second due to the limitations of the switching nodes.
Switching nodes consist of systems dedicated to switching the optical signals between lines as well as providing other signal processing functions, such as amplification and signal regeneration. Switching nodes include components such as optical switches, add/drop multiplexers, channel converters, routers, etc.
Prior art xe2x80x9copticalxe2x80x9d switches used in switching nodes are typically not entirely optical and therefore operate relatively slowly and have limited bandwidth. One type of known prior art switch is an opto-mechanical switch. Opto-mechanical switches use moving (e.g., rotating or alternating) mirrors, prisms, holographic gratings, or other devices to deflect light beams. The mechanical action may involve motors, or piezoelectric elements may be used for fast mechanical action. For example, Lucent Corporation and other companies have introduced a type of opto-mechanical switch referred to as a micro-electro-mechanical switch (xe2x80x9cMEMSxe2x80x9d). MEMS consist of arrays of actuated micro-mirrors etched onto a silicon chip in a similar manner to that of electrical integrated circuits. The mirrors change angle based upon an electrical signal and route an incident optical signal to one of many output fibers.
Another example of an opto-mechanical switch is a device from Agilent Technologies that steers optical signals through the controlled formation of gas bubbles within a liquid waveguide. A bubble is formed at the junction of one input and several output waveguides. The bubble will reflect an optical signal down one output while a lack of bubble will allow the signal to propagate through another waveguide.
A major limitation of opto-mechanical switches is low switching speeds. Typical switching times are in the millisecond range. The advantages of opto-mechanical switches are low insertion loss and low cross-talk.
Other prior art devices use electro-optic materials which alter their refractive indices in the presence of an electric field. They may be used as electrically controlled phase modulators or phase retarders. When placed in one arm of an interferometer, such as a Mach-Zender interferometer, or between two crossed polarizers, the electro-optic cell serves as an electrically controlled light modulator or a 1xc3x971 (on-off) switch. The most prevalent technology for electro-optic switching is integrated optics since it is difficult to make large arrays of switches using bulk crystals. Integrated-optic waveguides are fabricated using electro-optic dielectric substrates, such as Lithium Niobate (xe2x80x9cLiNBO3xe2x80x9d), with strips of slightly higher refractive index at the locations of the waveguides, created by diffusing titanium into the substrate. The major drawbacks of Lithium Niobate technology is the high expense of the material and difficulty in creating low loss waveguides within it.
Liquid crystals provide another technology that can be used to make electrically controlled optical switches. A large array of electrodes placed on a single liquid-crystal panel serves as a spatial light modulator or a set of 1xc3x971 switches. The main limitation is the relatively low switching speed.
Other prior art optical devices include acousto-optic switches which use the property of Bragg deflection of light by sound. An acoustic wave propagating along a dielectric surface alternatively puts the material in compression and tension. Thus, the acoustic pressure wave periodically alters the refractive index. The change in the refractive index is determined by the power of the acoustic wave, while the period of the refractive index change is a function of the frequency of the acoustic wave. Light coupled with the periodically alternating refractive index is deflected. A switching device can be constructed where the acoustic wave controls whether or not the light beam is deflected into an output waveguide.
Some prior art optical devices use magneto-optic materials that alter their optical properties under the influence of a magnetic field. Materials exhibiting the Faraday effect, for example, act as polarization rotators in the presence of a magnetic flux density B. The rotary power xcfx81 (angle per unit length) is proportional to the component B in the direction of propagation. When the material is placed between two crossed polarizers, the optical power transmission T=sin2"THgr" is dependent on the polarization rotation angle "THgr"=xcfx81d where d is the thickness of the cell. The device is used as a 1xc3x971 switch controlled by the magnetic field.
Finally, prior art optical devices do exist that can be considered xe2x80x9call-opticalxe2x80x9d or xe2x80x9coptic-opticxe2x80x9d switches. In an all-optical switch, light controls light with the help of a non-linear optical material. They operate using non-linear optical properties of certain materials when exposed to high intensity light beams (i.e., a slight change in index under high intensities).
FIG. 1 illustrates a prior art all-optical switch 20 that uses an interferometer. Switch 20 includes material 14 that exhibits the optical Kerr effect (the variation of the refractive index with the applied light intensity) which is placed in one leg of a Mach-Zender interferometer. An input signal 10 is controlled by a control light 16. As control light 16 is turned on and off, transmittance switch 20 at output 12 is switched between xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d because the optical phase modulation in Kerr medium 14 is converted into intensity modulation.
FIG. 2 illustrates a prior art all-optical switch 30 that uses an optical loop. Switch 30 is a non-linear optical loop mirror (xe2x80x9cNOLMxe2x80x9d) that includes a fused fiber coupler (splitter) 34 with two of its arms connected to an unbroken loop of fiber 32. A signal arriving at the input 36 to coupler 34 is split and sent both ways around fiber loop 32. One of the lengths of loop 32 contains a Kerr medium. The Kerr medium is pumped via another high intensity control beam that alters the refractive index of the material and thus slightly changes the speed at which the signal beam propagates through. When the two signal beams recombine at the other end of loop 32 interference effects determine the amplitude of the output 38. Although a NOLM operates at high speeds (tens of picoseconds), it requires long lengths of fibers and is not readily integratable.
The retardation between two polarizations in an anisotropic non-linear medium has also been used for switching by placing the material between two crossed polarizers. FIG. 3 illustrates a prior art all-optical switch 30 using an anisotropic optical fiber 42 that exhibits the optical Kerr effect. In the presence of a control light 43, fiber 42 introduces a phase retardation xcfx80, so that the polarization of the linearly polarized input light 45 rotates 90xc2x0 and is transmitted at output 46 by an output polarizer 48. In the presence of control light 43, fiber 42 introduces no retardation and polarizer 48 blocks input light 45. A filter 44 is used to transmit input light 45 and block control light 43, which has a different wavelength.
FIG. 4 illustrates another prior art all-optical device 50 that uses liquid-crystal. Device 50 includes an array of switches as part of an optically addressed liquid-crystal spatial light modulator 52. A control light 54 alters the electric field applied to the liquid-crystal layer and therefore alters its reflectance. Different points on the liquid-crystal surface have different reflectances and act as independent switches controlled by input light beams 58 and output as output light beams 56. Device 50 can accommodate a large number of switches, but is relatively slow.
Still another all-optical switch is based on an optically pumped Semiconductor Optical Amplifier (xe2x80x9cSOAxe2x80x9d). A SOA is a laser gain medium without a resonator cavity. A SOA-based switch operates similar to the NOLM in that it operates on interference between laser beams. Like the NOLM, SOA-based devices have significant loss and require high operating power. They also suffer other non-linear effects including frequency addition, which has the effect of switching the data to a different wavelength channel. These detriments have prevented SOAs from being commercially viable.
Based on the foregoing, there is a need for an all-optical switch having low power requirements and fast switching speeds.
One embodiment of the present invention is an optical switch that includes a saturable absorber. Coupled to the saturable absorber is an input waveguide, an output waveguide, and a control beam waveguide.